Overall Water Splitting by a SrTaO2N-Based Photocatalyst Decorated with an Ir-Promoted Ru-Based Cocatalyst

The development of narrow-bandgap photocatalysts for one-step-excitation overall water splitting (OWS) remains a critical challenge in the field of solar hydrogen production. SrTaO2N is a photocatalytic material having a band structure suitable for OWS under visible light (λ ≤ 600 nm). However, the presence of defects in the oxynitride and the lack of cocatalysts to promote simultaneous hydrogen and oxygen evolution make it challenging to realize OWS using this material. The present work demonstrates a SrTaO2N-based particulate photocatalyst for OWS. This photocatalyst, which was composed of single crystals, was obtained by nitriding SrCl2 and Ta2O5 together with NaOH, with the latter added to control the formation of defects. The subsequent loading of bimetallic RuIrOx nanoparticles accelerated charge separation and allowed the SrTaO2N photocatalyst to exhibit superior OWS activity. This research presenting the strategies of controlling the oxygen sources and promoting the cocatalyst function is expected to expand the range of potential OWS-active oxynitride photocatalysts and permit the design of efficient cocatalysts for photocatalytic OWS.

O ne-step-excitation overall water splitting (OWS) using sunlight and a particulate photocatalyst offers a simple route to the realization of sustainable hydrogen production. 1,2ecently, solar hydrogen production on the 100 m 2 scale has been achieved based on OWS with an Al-doped SrTiO 3 particulate photocatalyst, thus demonstrating the feasibility of this process on a large scale. 3Nevertheless, the solar-tohydrogen (STH) energy conversion efficiency of this 100 m 2 panel system was only approximately 0.76%. 3In fact, increasing the STH efficiency of such systems beyond 1.7% is considered impossible because the photocatalyst responds solely to ultraviolet light. 1 Obtaining an STH efficiency of 5%, which is the lowest value at which this process becomes practical, will require the development of the OWS photocatalysts having band gap energies, E g , below 2.1 eV. 1 To date, InGaN-based nanorod arrays directly grown on a substrate have demonstrated STH of 3−5%. 4,5However, only a few particulate photocatalysts, such as LaMg 1/3 Ta 2/3 O 2 N, 6,7 Ta 3 N 5 , 8 Y 2 Ti 2 O 5 S 2 , 9 and BaTaO 2 N, 10 have been reported to have E g ≤ 2.1 eV and to drive OWS, and the apparent quantum yield (AQY) under visible light was at most 0.36%. 9rTaO 2 N (E g = 2.1 eV) is a perovskite-type oxynitride photocatalyst with a band structure suitable for promoting both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) from aqueous solutions under visible light.To date, SrTaO 2 N has been applied to drive either the HER or OER individually in the presence of sacrificial reagents 11,12 but has not been successfully applied to OWS.This is partly due to the inevitable presence of defects in SrTaO 2 N that enhance charge recombination.Another crucial factor is the lack of cocatalysts to promote charge separation and simultaneous HER and OER.The dual cocatalyst strategy in which both a hydrogen evolution cocatalyst (HEC) and oxygen evolution cocatalyst (OEC) are incorporated into the OWS system has become popularly investigated to promote OWS effectively. 10,13,14However, the possible HEC candidates are presently limited to certain noble metals. 2 Bimetals and bimetallic oxides may provide enhanced activities compared with monometallic cocatalysts owing to the tunability of physicochemical properties. 15,16Therefore, it would be desirable to explore the design of simple yet effective methods for synthesizing SrTaO 2 N with few defects and loading bimetallic metal or metal-oxide nanoparticles on photocatalyst surfaces jointly.
The present work demonstrates the application of a SrTaO 2 N-based particulate photocatalyst to promote OWS.The direct nitridation of SrCl 2 and Ta 2 O 5 in the presence of NaOH produced single-crystal SrTaO 2 N particles with few defects and byproducts.Loading nanoparticles of the bimetallic RuIrO x with the CrO y shell on this material allowed the SrTaO 2 N photocatalyst to exhibit superior OWS activity compared with other perovskite oxynitride photocatalysts.
SrTaO 2 N was synthesized by the nitridation of SrCl 2 , Ta 2 O 5 , and NaOH combined in a 4:1:n molar ratio.Here, the SrCl 2 served as the Sr source as well as the flux, while the NaOH served as the O source and the flux.This material is referred to herein as SrTaO 2 N(n), where n is the NaOH/Ta 2 O 5 molar ratio.
The X-ray diffraction (XRD) patterns in Figure 1A indicate that a combination of SrTaO 2 N and Ta 3 N 5 phases was formed in the case of SrTaO 2 N(0).The amount of Ta 3 N 5 in the product was decreased when adding NaOH during the synthesis of the SrTaO 2 N(1) specimen.Additional characterization by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy (XPS) indicated that SrTaO 2 N(1) was not doped with Na (Figure S1a).The weight fractions of Sr, Ta, O, and N were measured to be 23.0, 61.1, 5.3, and 8.5%, respectively, indicating that the sample consisted of 0.78(SrTaO 2 N) + 0.07(Ta 3 N 5 ).The data acquired using UV−visible diffusereflectance spectroscopy (DRS) as presented in Figure 1B and Figure S2B demonstrate that the optical absorption edge for each SrTaO 2 N(n) sample was at approximately 600 nm regardless of the value of n.An absorption background associated with defects in the photocatalyst is also observed beyond the absorption edge.Notably, these features related to defects and byproducts became weaker with an increasing amount of NaOH, and pure SrTaO 2 N was obtained at n = 2 (Figure S2).These results suggest that adding appropriate amounts of NaOH can reduce the defect density of SrTaO 2 N and the amount of the Ta 3 N 5 byproduct.
The selected area electron diffraction (SAED) pattern and the matching of the lattice spacings of SrTaO 2 N(1) with those of a reference SrTaO 2 N crystal structure 12 suggest the formation of single-crystal SrTaO 2 N nanoparticles (Figures 1C and D).The average size of the cube-like SrTaO 2 N(1) particles was determined to be 124 nm (Figures S1b and S1c).Rod-like particles were also observed in high-resolution transmission electron microscopy (HR-TEM) images.These particles were found not to include Sr species based on analyses by energy dispersive X-ray spectroscopy (EDS) and therefore attributed to Ta 3 N 5 (Figure S3).SrTaO 2 N(1) sequentially loaded with IrO 2 by microwaveassisted heating (denoted as IrO 2(MW) ), Ru by impregnation-H 2 reduction, and CrO y by photodeposition (see the Supporting Information for details) were capable of evolving H 2 and O 2 from water simultaneously under visible light (λ > 420 nm) with initial evolution rates of 9.1 and 3.0 μmol h −1 , respectively (Figure 2A).The deviation from the stoichiometric ratio of the two was due to low quantification accuracy near the detection limit of O 2 (Figure S4A).The optimal nominal Ir, Ru, and Cr contents were 1, 4, and 4 wt %, respectively (Table S1), while the actual loading proportions were found to be 0.8, 4.0, and 0.6 wt %, respectively, by ICP-AES.The AQY in response to irradiation at 420 ± 30 nm was determined to be 0.34% in the initial stage of the reaction.A slight loss in the OWS activity was observed after three repeated uses of the photocatalyst, after which the performance remained essentially constant with further use.The STH value was measured to be 5−6 × 10 −3 % over the 48-h reaction (Figure S4B).SrTaO 2 N(0) prepared in the absence of NaOH exhibited lower OWS activity because of higher defect density and impurity Ta 3 N 5 content (Figure S2C).On the other hand, SrTaO 2 N(2) prepared with a larger NaOH amount also exhibited lower OWS activity even though it comprised singlephase SrTaO 2 N. Notably, single-phase SrTaO 2 N was also obtained by using SrCO 3 instead of NaOH (Figure S5), but it could not split water in this study.These results suggest the importance of controlling the amount of oxygen sources to produce stoichiometric and active SrTaO 2 N. The optimal NaOH/Ta 2 O 5 molar ratio in the original formulation was unity.
The cocatalysts evidently had a significant effect on the OWS activity.The OWS activity decreased without the loading of IrO 2(MW) (Figure 2B).Adsorption of colloidal IrO 2 (denoted as IrO 2(AD) ) before or after the loading of the CrO y /Ru cocatalyst was not effective in enhancing OWS although IrO 2(AD) effectively promoted the reaction on Y 2 Ti 2 O 5 S 2 , 9 BaTaO 2 N, 10 and TaON. 14Therefore, an indepth understanding of the structures and functions of the CrO y /Ru/IrO 2(MW) cocatalyst is imperative.
The annular dark-field scanning TEM (ADF-STEM) images of the CrO y /Ru/IrO 2(MW) /SrTaO 2 N(1) specimen in Figure 3 indicate that the Ru particle size in the present samples was around 5 nm.The EDS images show that typical core (Ru species)-shell (Cr species) nanostructures were obtained.−10 The XRD pattern for the Ru/IrO 2(MW) /SrTaO 2 N(1) suggests the copresence of metallic Ru and RuO 2 (Figure S6).The latter compound likely resulted from surface oxidation of Ru 0 nanoparticles, in agreement with our previous results. 14On the other hand, the Ir species showed weak signals distributed over the entire SrTaO 2 N(1) particles in STEM-EDS analysis, and their structure was unclear.
To gain the information about the chemical states, the Ir species loaded on SrTaO 2 N(1) was analyzed by XPS (Figure 4A).The Ir species in IrO 2(MW) /SrTaO 2 N(1) was attributed to IrO 2 on the basis of the Ir 4f 7/2 and 4f 5/2 signals at binding energies of 62.2 and 65.2 eV, respectively. 17After the H 2 reduction, the Ir peaks shifted to lower binding energies, indicative of partial reduction to metallic Ir.The fraction of metallic Ir became greater in the presence of Ru to produce the Ru/IrO 2(MW) cocatalyst.Concurrently, the fraction of metallic Ru was decreased by the presence of IrO 2(MW) compared with the sample without Ir, as shown in the Ru 3p XPS analysis (Figure S7).The opposite movements of the Ir and Ru peaks suggest that there were certain electronic interactions between them in the Ru/IrO 2(MW) cocatalyst owing to the formation of the RuIr bimetal and RuIrO 2 bimetallic oxide nanoparticles (denoted as RuIrO x for simplicity). 18Taking the additional EDS mapping results (Figure S8) into account, it can be concluded that RuO x , RuIrO x , and IrO x nanoparticles were primarily present on the SrTaO 2 N(1) surface.
The roles of IrO x , RuO x , and RuIrO x cocatalysts during photocatalytic OWS were further assessed by transient absorption spectroscopy (TAS) (Figure S9). 19Probing at a wavelength of 5000 nm (∼0.25 eV) upon band gap excitation of SrTaO 2 N(1) by a 470 nm pump was used to monitor the decay behavior of electrons (Figure 4B).When only RuO x was loaded on SrTaO 2 N(1) (i.e., Ru/SrTaO 2 N(1)), the intensity of the absorption at 5000 nm increased, and the decay was notably slower compared with that observed for bare SrTaO 2 N(1).These data demonstrate that Ru/RuO x served not only as an HEC but also as an OEC and captured photogenerated holes in SrTaO 2 N(1).These outcomes agree with the observed promotion of the sacrificial HER and OER activities upon loading Ru onto SrTaO 2 N(1) (Figure S10).When Ir was included in RuO x (i.e., Ru/IrO 2 (MW) /SrTaO 2 N-(1)) to form the new cocatalyst RuIrO x , the decay of electrons in SrTaO 2 N(1) was greatly accelerated.Considering the notable enhancement in the sacrificial HER and OER activities compared with the Ru/SrTaO 2 N(1) specimen (Figure S10), this implies that RuIrO x captured electrons very efficiently from SrTaO 2 N and worked as an active HEC.Moreover, the TAS results (Figure S11) for IrO 2(MW) /SrTaO 2 N(1) subjected to the reduction treatment indicate both electrons and holes could be captured by IrO x , leading to enhanced charge recombination.Collectively, RuIrO x , having significant electron capture capacity, with a CrO y shell worked as an active HEC, while RuO x served as a passable OEC (Figure 4C).The IrO x species present in minor amounts most likely formed recombination centers that were detrimental to the OWS.
In summary, this work established a SrTaO 2 N-based onestep-excitation OWS system.By reducing the defect density in SrTaO 2 N based on adding NaOH and by the formation of RuIrO x cocatalysts that promoted charge separation and surface reactions, this OWS system achieved an STH value of 6.3 × 10 −3 % and an AQY (420 ± 30 nm) of 0.34%.These are one of the highest values yet reported for a perovskite oxynitride photocatalyst with E g ≤ 2.1 eV.RuIrO x and RuO x were found to work as an HEC and OEC, respectively.Further activity improvements are expected by controlling the morphology of the photocatalyst particles to make facetselective deposition of these cocatalysts feasible. 20Controlling the oxygen source during the nitridation and promoting the cocatalyst function by forming bimetallic systems will help the development of other semiconductor/cocatalyst systems for OWS.

Figure 1 .
Figure 1.(A) XRD patterns and (B) DRS spectra for (a) SrTaO 2 N(0) and (b) SrTaO 2 N(1).(C) TEM with SAED and (D) ADF-STEM of a cross-section of the SrTaO 2 N(1) sample.The blue circle and red square indicate the areas assessed by SAED and ADF-STEM, respectively.